Process for the production of a piece of jewellery

ABSTRACT

A process for the production for a piece of jewellery by casting metal around a precious stone, wherein prior to the casting operation the precious stone is provided with a colouring agent-bearing coating and that during or after the application of the coating, the latter is subjected to an energetic treatment which causes those transformations of the coating which would occur without the energetic treatment upon casting the metal around the precious stone.

[0001] The invention relates to a process for the production of a piece of jewellery by casting metal around a precious stone.

[0002] A process of that kind is also suitable in particular for stones which are relatively good value for money, in which the separate production of a part of the piece of jewellery from precious metal and the fact of joining it to the stone by a goldsmith signify a relatively high level of expenditure.

[0003] The precious stones for use in the process of the general kind set forth are in particular stones whose natural colour has been artificially enhanced or altered. There are two methods available for altering the colour of the stone, and a basic distinction is to be drawn between them: it is possible to cause colouring elements to diffuse into the stone or however it is possible for a coloured layer, for example a pigment-bearing layer, to be fixedly joined to the stone.

[0004] It is only the second kind of colour alteration that is of interest here as it is substantially more suitable for industrial mass production. For example it is known from U.S. Pat. No. 3,837,884 (Balzers) for a blue colour layer to be applied to a main body by vapour deposition of cobalt oxide, aluminium oxide and silicon oxide. Although those layers are tempered for a period of about 30 minutes at about 300° C. in an oxygen-bearing atmosphere, they are not stable under the conditions of the process of the general kind set forth.

[0005] It was surprisingly found that precious stones which are provided with a colour coating thereon can be processed in a casting process if it is provided that, during or after the application of the coating, the latter is subjected to an energetic treatment which causes those transformations of the coating which would occur without the energetic treatment upon casting the metal around the precious stone.

[0006] In the production of a piece of jewellery, as defined in the opening part of this specification, the stone is firstly exposed for a relatively long period of time to that temperature at which the wax pattern which finally is to be replaced by the cast-in precious metal is burnt out. That temperature is of the order of magnitude of 750° C. Then, when the precious metal is being poured in, the surface of the stone is heated to a temperature which is between 950° and 1050° in the case of silver, between 950° and 1250° in the case of gold and between 1800° and 1900° C. for platinum. As is to be discussed hereinafter with reference to the embodiments, it is sufficient if the stones provided with a colour layer are briefly heated to the temperature at which the metal is poured in and/or are heated to the temperature for burning out the wax pattern over a relatively long period of time, in order to reduce to a minimum the colour changes which still occur in the pouring-in operation itself.

[0007] Further features of the invention will now be described in greater detail with reference to the drawing and the embodiments by way of example.

[0008]FIGS. 1-3 are diagrammatic view of three conventional phases in the process according to the invention, and

[0009]FIG. 4 is a view in section through a detail of a piece of jewellery produced in accordance with the invention.

[0010] The production of a piece of jewellery by casting metal around a precious stone is known per se. As shown in FIG. 1 the stone 1 is embedded in mould material 5, whereas firstly wax 6 serve as a place-holder for the metal of the stone setting. Then the wax 6 is expelled by heating to about 750° C., thus resulting in the hollow space 7 shown in FIG. 2. That hollow space 7 is finally filled with metal 4 which surrounds the stone 1, as shown in FIG. 3.

[0011] As an example of a piece of jewellery, FIGS. 1-3 show a ring but the invention can be used wherever the surface of a stone, in particular the rear side 2 thereof (see FIG. 4), is provided with a coloured coating which could be adversely affected by pouring in metal 4.

[0012] A prerequisite for application of the process according to the invention is firstly that the stone retains its optical properties even when metal is cast therearound. Glass and quartz for example can be readily provided with a heat-resistant colour layer, but normal glass would melt at the specified temperatures. Glass would already become cloudy at the temperatures which are usual for burning out the wax.

[0013] A basically suitable stone, for example zirconium oxide, is now provided with a colour layer. The most common methods for that purpose are vapour deposition, sputtering, chemical vapour deposition and so forth. Specifically by reactive sputtering onto a slightly heated substrate, it is possible directly to apply most organic colours even at low temperatures.

[0014] In particular dyestuffs whose basic compositions are known as inorganic coloured pigments, in particular inorganic materials which have a “streak”, are considered for colouring the stone. Such dyes are known in connection with ceramic glazes and with surface colouring of glass. Colours layers based on oxidic pigments with a rutile, cassiterite, zircon, baddeleyite, corundum, haematite, olivine, spinel, gamet and perovskite structure are particularly suitable. Colour layers based on nitridic and more especially oxinitridic pigments also appear to be highly suitable for the process as well as colour layers based on carbides and carboxides. Colour layers based on sulphides, sulphates, phosphates, cyanogen components as well as metal colloids need special embedding but they are suitable even when they have a melting point in the proximity of or just below the pouring-in temperature.

[0015] If a stone prepared in that way were to be further processed in the procedure shown in FIGS. 1-3, that would result in unwanted and substantially uncontrolled colouration of the stone. Such colouration effects are quantified in accordance with the “Colored Stone Grading System” of the Gemological Institute of America. In that respect colour impression (hue), tone (light/dark) and colour saturation are assessed. The effect achieved by the invention provides that stones which, without particular post-treatment of the coloured coating, in accordance with at least one of the specified criteria, would experience a change by more than two points in the pouring-in operation, now exhibit a deviation of at most two points, normally at most one point. That is achieved by an energetic treatment which anticipates the loading which occurs when burning out the wax and when pouring in the metal.

[0016] Set out hereinafter are some examples of carrying the process according to the invention into effect. In this respect designation of the pigments used is implemented by specifying the CAS registration number of the American Chemical Society. Pigments suitable for high temperatures are specified in Ullmann's Encyclopedia of Industrial Chemistry 1986, Vol. A5, in particular pages 546/547. Those dyes can all be produced in accordance with the invention either directly or by the selection of a suitable intermediate, embedding or cover layer, in such a way that they no longer change their colour in the pouring-in operation.

EXAMPLE 1 (COMPARATIVE TEST)

[0017] A layer which consisted for about 35% by weight of cobalt oxide, 10% of aluminium oxide and the balance silicon oxide, was applied by vapour deposition to cubic zirconia, in accordance with the process described in U.S. Pat. No. 3,837,884. A subsequent heat treatment produced blue colouration of the layer. Casting in the stone provided with the coloured layer, without post-treatment, resulted in discolouration of the stone at the pouring temperature of 14 ct yellow gold (about 1050° C.).

[0018] In accordance with the invention the coated stone was heated for 10 minutes at 1200° C. and then tempered for an hour at 750° C. (this corresponds to the wax burning-out temperature). By virtue of the described heat treatment, no subsequent changes occurred, not even at the pouring-in temperature of 14 ct Pd-white gold (1250° C.).

EXAMPLE 2 (COMPARATIVE TEST)

[0019] A pigment of the composition CAS 68186-85-6 (green spinel, Co₂TiO₄) with some TiO₂ in excess was applied to zirconia by reactive sputtering at low temperature. When pouring in 14 ct palladium-white gold, a markedly visible change in the green colour occurred.

[0020] If in contrast the described layer was tempered for a minute at 1200° C., the coated stone exhibited no noticeable colour change when the metal was poured in.

[0021] If in contrast the stone tempered at 1200° C. was cast into a 14 ct yellow gold melt, nonetheless a brownish change in colour occurred. The change in colour was at the greatest at the edge where the colour layer was in direct contact with the alloy.

[0022] If, in addition to the colour layer, a protective layer of magnesium aluminate was also applied and the stone tempered at 1200° C., the colour shade did not change, even at the locations at which there was direct contact with the hot yellow gold alloy.

EXAMPLE 3

[0023] A colour layer based on the pigment CAS 68187-35-9 (brown haematite, Fe₂O₃) was produced in temperature-stable form and chemically and mechanically resistant, by a procedure whereby the iron oxide pigment was applied, jointly with an embedding matrix of aluminium oxide, to a thin SiO₂ layer. The treatment for 10 minutes at 1200° C. was followed by a post-treatment for two hours at 750° C. The colour shade of the pigment embedded and heat-treated in that way admittedly no longer precisely corresponds to the colour of the pure pigment, but on the other hand the colour did not change in the pouring-in operation.

EXAMPLE 4

[0024] Zirconia was coated by means of the pigment CAS 68187-05-3, a blue-green spinel with the basic formula Co₂SnO₄. The pigment layer which was about 125 mm was coated with a dense protective layer of aluminium oxide and heated for a minute at 1200° C. Subsequently the stone was tempered for 30 minutes at 750° C. in air. The resulting blue colouration was maintained when pouring in sterling silver.

[0025] In individual ones of the specified examples, account was taken of the fact that the coloured coating on a precious stone can be damaged not only by the temperature which occurs when pouring in metal. Rather, unwanted changes can also occur at the interface between the stone and the colour layer and in particular at the outside of the colour layer, and those unwanted changes can be avoided by separation layers. In particular that takes account of the fact that the shine can suffer even in the case of a colour-fast stone when pouring in metal.

[0026] If there is a wish to also safeguard the colour-coated rear side of a stone against mechanical attack, in particular against the impacting action of grinding members or other pieces of jewellery, as well as the abrasive effect of diamond grinding grains, then, as shown in FIG. 4, it can be provided that the rear side of the precious stone is surrounded by a ring formed by the poured-in metal. 

1. A process for the production of a piece of jewellery by casting metal around a precious stone, characterised in that prior to the casting operation the precious stone is provided with a colouring agent-bearing coating and that during or after the application of the coating, the latter is subjected to an energetic treatment which causes those transformations of the coating which would occur without the energetic treatment upon casting the metal around the precious stone.
 2. A process according to claim 1 characterised in that the energetic treatment includes heating to a temperature of between 950° C. and 1250° C. for a period of 1 to 20 minutes.
 3. A process according to claim 2 characterised in that the energetic treatment includes a tempering operation at a temperature of between 700° and 800° C. for a period of at least one hour.
 4. A process according to claim 1, characterised in that the colouring agent-bearing coating is provided with a protective covering.
 5. A process according to claim 4 characterised in that Li, Mg, Zn, B, Ce, Ti, Si, Zr or Al is contained in the covering.
 6. A process according to claim 1 characterised in that a transitional layer is arranged between the precious stone and the coloured coating.
 7. A process according to claim 1 characterised in that the rear side (2) of the precious stone (1) is protected by a ring (3) formed by the poured-in metal (4).
 8. A precious stone which is suitable for the production of an article of jewellery by pouring metal therearound according to claim 1 characterised in that the precious stone is provided with a colouring agent-bearing coating of per se variable colour, which no longer substantially changes by virtue of an energetic treatment upon heating to a temperature below 1250° C. 